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Doppler Effect: Frequency, Pitch and Volume.

If you are not entirely sure of how pitch and the Doppler effect are related, this one is for you.

The Doppler effect describes how the pitch heard from moving sources of sound such as planes, trains or automobiles varies (predictably) depending on whether the source is approaching, or receding. Or - is it the volume that changes? When a source of sound comes closer, surely it also gets louder and its volume also changes.

It is not usually pointed out in descriptions of the Doppler effect, but it is a fact that when a source of sound approaches or recedes, both the volume of sound as well as its pitch, change. If you are not sure what is what, you may confuse one with the other, and end up not fully appreciating Herr Doppler's simple yet brilliant insight.

Let us back up a little. When an object is struck with some force, the energy behind the force is transmitted to the object in a few different ways. For example, it is manifested as mechanical vibrations and as heat. Right now, we're interested in the vibrations. A surface that is vibrating and is in contact with a medium such as air (or water, or anything that is not a vacuum), also makes the medium vibrate at approximately the same rate. These vibrations - layers of air being compressed and released in synch with the vibrating surface, like an accordion - travel outward and are heard as "sound" when they reach our eardrums.

The number of times per second the surface vibrates (and by extension the air around it), is called the frequency. Frequency is an objective physical quantity that can be measured independently by one or more instruments, and can be validated across readings.

However, frequency is perceived by human ears as pitch, which is a subjective quantity. In other words, pitch depends on the observer and is not the same for all observers. Pitch is the inner human sensation that is produced in response to the frequency sound, so there are many factors that go into it. Doppler's effect is about the perception of sound by human beings, so it is about pitch. When a sound source is travelling towards you, each successive vibration that reaches your ears travels a shorter distance, and this has the effect of making the pitch rise.

A rising pitch is what you hear when the guitarist slides from the nut (where the tuning keys are located), towards the bridge. You hear a decreasing pitch when he slides in the opposite direction. Spend some time listening to this, and understanding how to identify a rising pitch and a falling pitch. In this example the volume remains the same, so you can also practice separating those two concepts out.

The next time you hear an approaching ambulance or plane, realize that its volume is rising but so is its pitch, and focus on that rising note to understand what the Doppler effect describes. Sound sources that are receding register a lowering pitch. A fascinating extension of the Doppler effect is found in electromagnetic waves; light sources that are travelling at speeds comparable to that of light, register similar shifts at the ends of the spectrum of color, and allow astronomers to calculate their velocity.